Western Australia in May 2013: warm nights and above average rainfall

Rainfall

Rainfall during May 2013 was above to very much above average in the Kimberley
and much of the Pilbara and Northern Interior, as well as parts of the Southern
Interior, Goldfields, Gascoyne, Southwest, Southeast Coastal, and west Eucla.
Theda and Camballin in the Kimberley, and Balladonia in the western Eucla recorded
their wettest May on record, largely due to rainfall events toward the middle
and second half of the month. For the remainder of the State, May rainfall was
generally close to average.

Averaged across the state as a whole, rainfall during May 2013 was above average.
For the Southwest Land Division (SWLD) and the Lower Southwest (southwest of
a line from Jurien Bay to Bremer Bay), May 2013 rainfall was close to, but slightly
above, the May average, resulting in their first 'above average' May since 2005.

May 2013 started with a cloudband moving through the SWLD and the adjacent
Gascoyne, Goldfields, and Eucla from the 1st to the 3rd, producing widespread
rainfall and isolated thunderstorms with moderate to heavy falls recorded in
the Southwest and southern parts of the Lower West.

A strong cold front moved across western WA during the evening of the 7th and
morning of the 8th, resulting in thunderstorms, severe wind gusts, and widespread
rainfall in western, central and southern WA. The heaviest falls were observed
in the Southwest and Lower West districts with daily totals mostly in the 30 mm
to 90 mm range, while moderate daily totals extended into the Central West,
Gascoyne and west Pilbara. A very heavy fall of 100.8 mm in the 24 hours
to 9am on the 8th at Aston Downs Alert (Southwest) was the highest daily total
at the site in 10 years of record, and the highest daily total for May 2013
in the SWLD, while numerous other sites with mostly short periods of record
observed their wettest May day on record. The cold front weakened and moved
through southeastern WA on the 9th resulting in moderate rainfall in the Goldfields
and Eucla, while two fronts passed the SWLD in succession on the 9th and the
10th, bringing further showers and isolated thunderstorms, severe winds gusts
and moderate rainfall to the Southwest.

A cloudband associated with a cold front over southeast Australia produced
rain and isolated thunderstorms with moderate rainfall in the Kimberley on the
12th and early on the 13th, whilst a middle level trough brought widespread
rainfall to the Pilbara and west Kimberley on the 14th and 15th, with thunderstorms
and heavy falls in the west Kimberley on the 15th. Storms persisted in the Kimberley
to the 24th with further heavy falls reported. Very heavy rainfall in excess
of 150 mm was reported on the northern Dampier Penninsula on the 19th and
20th as Cygnet Bay observed 155.4 mm in the 24 hours to 9am on the 21st,
its wettest May day in 50 years of record. Lombadina Airstrip reported 182.5
mm on the 21st, the highest daily total in WA for May 2013.

Showers and thunderstorms produced moderate to heavy rainfall in the west Pilbara
on the 26th and the 27th, causing localised flash flooding around Exmouth where
a daily fall of 106.0 mm was reported at Exmouth Town in the 24 hours to
9am on the 27th.

A cold front moved across the southwest of WA overnight on the 29th and moved
through central and southeastern WA on the 30th and 31st, bringing thunderstorms
and moderate to heavy rainfall to western and southern parts of the SWLD. Particularly
heavy falls were observed along the Southeast Coastal coast, with Esperance
and Mt Howick observing their wettest May day in 44 and 40 years of record respectively
with daily totals near 50 mm.

Maximum temperature

Much of WA recorded near average maximum temperatures in May 2013. Above to
very much above average mean maximum temperatures were observed in most of the
Kimberley, Southern Interior, and Eucla, with an area up to 4 °C warmer
than normal in the eastern Eucla. Below average mean maximum temperatures were
mainly confined to small areas in the the west Pilbara, inland Gascoyne, and
Northern Interior.

Averaged across the state as a whole, the mean maximum temperature for May
was near average for both WA and the Lower Southwest (southwest of a line from
Jurien Bay to Bremer Bay).

During the first eight days of the month, unusually warm days were experienced
in various locations in northern, central, and southeastern WA with daily maxima
reaching the high thirties at times due to a number of slow moving troughs directing
hot northerly winds over much of WA. Onslow Airport in the west Pilbara recorded
a maximum of 38.8 °C on the 1st, its hottest May day in 48 years of record.
Forrest in the Eucla district recorded a maximum temperature of 36.8 °C
on the 2nd breaking its previous highest May temperature in 83 years of record
of 36.6 °C on 3 May 1990, whilst Eucla town recorded a maximum temperature
of 36.5 °C on both the 2nd and 8th, exceeding its previous highest
May temperature (36.3 °C) in 52 years of record. Numerous other sites with
shorter periods of record also registered their highest May temperature. Mandora
in the west Kimberley recorded the highest temperature in WA in May with a maximum
of 39.2 °C on the 6th.

In the last two days of the month, a strong cold front and a pool of cold air
moved over southern WA and, along with cloud and rain, caused very low temperatures
in the SWLD. Maximum temperatures below 10 °C were observed at a number
of sites on the 30th as Jacup and Manjimup observed the lowest maximum temperature
of the month in WA with 9.4 °C, and Ongerup registered its lowest maximum
temperature in May in 48 years of record with 9.8 °C.

Minimum temperatures

Most of WA was above to very much above average in terms of mean minimum temperatures
during May 2013, with means up to 3 °C warmer than normal in the west
and central Kimberley and Southern Interior, grading to 4 °C warmer
than normal in parts of the Eucla. Large parts of western WA saw near average
overnight temperatures, with a small area below average in the west Gascoyne.

Averaged across the state as a whole, the mean minimum temperature for May
2013 was the 9th warmest since comparable records commenced in 1910. Averaged
across the Lower Southwest (southwest of a line from Jurien Bay to Bremer Bay),
the mean minimum temperature in May 2013 was above average.

Warm northerly winds and cloud resulted in some very warm nights in northern,
central, and eastern parts of WA during the first eleven days of the month.
Balladonia in the western Eucla recorded a minimum temperature of 18.8 °C
on the 2nd, which exceeded its previous May record of 17.5 °C set on
13 May 1972. Mardie in the Pilbara recorded a minimum of 25.1 °C on the
8th, its warmest May night in 56 years of record, whilst Giles in the Southern
Interior recorded a minimum of 23.3 °C on the 9th, its warmest May night
in 57 years of record.

A cold morning was experienced across the SWLD on the last day of May 2013
as mimimum temperatures dropped to below 5 °C at numerous sites and
a number of sites registered sub-zero temperatures. A minimum of 3.3 °C
at Lancelin (Lower West) on the 31st was its lowest May temperature in 46 years
of record, whilst Collie East recorded a minimum of -0.6 °C, the lowest
temperature recorded in WA in May 2013.

Notes

A Monthly Climate Summary is prepared to list the main features of the weather in Western Australia using the most timely and accurate information available on the date of publication; it will generally not be updated. Later information, including data that has had greater opportunity for quality control, will be presented in the Monthly Weather Review, usually published in the fourth week of the month.

This statement has been prepared based on information available at
3 pm on Monday 3 June 2013.
Some checks have been made on the data, but it is possible that results will change
as new information becomes available.

Averages are long-term means based on observations from
all available years of record, which vary widely from site to site.
They are not shown for sites with less than 10 years of record, as they cannot then be calculated reliably.
The median
is sometimes more representative than the
mean
of long-term average rain.

The Rank indicates how rainfall this time compares with the climate record for the site,
based on the
decile ranking
(very low rainfall is in decile 1, low in decile 2 or 3,
average in decile 4 to 7, high in decile 8 or 9
and very high is in decile 10).
The Fraction of average shows how much rain has fallen this time as a
percentage of the long-term mean.

The ACORN-SAT dataset
is being used for temperature area averages from December 2012 onwards.
The major change from earlier datasets is that the ACORN-SAT dataset commences in 1910, rather than 1950,
and hence rankings are calculated using a larger set of years.